Wumpus, q, is a reference to the old game called Hunt the Wumpus. It is a very slow but strong animal that can cling to ceilings. This attribute allows it to be generated over and move through water without drowning or being relocated. It is a rather rare creature but not among the rarest. Should you ascend a game, you are pretty much guaranteed to meet at least a few wumpuses.

Wumpuses generated as part of a level usually start off asleep (80% chance), though this does not apply after you have picked up the Amulet of Yendor or to wumpuses generated after level creation.[1]

Pet wumpuses can be ridden while on land, but you will fall off the wumpus when you try to ride over water, since they cross water by walking on the ceiling. To be more accurate, you fall into the water, wetting your inventory and possibly drowning. If you don't drown, you end on an attached land square, and the wumpus on the water square you tried to ride on. The saddle stays on the wumpus.

The ranger quest locate level is a homage to Hunt the Wumpus and has a guaranteed wumpus.

Encyclopedia entry

The Wumpus, by the way, is not bothered by the hazards since
he has sucker feet and is too big for a bat to lift. If you
try to shoot him and miss, there's also a chance that he'll
up and move himself into another cave, though by nature the
Wumpus is a sedentary creature.